subtests.docker_cli.tag: Bugfixes and style cleanup

* --force was impossible to set due to config automatic translation
* exit_code in FAIL branch was incorrectly controlled
* use NoFailDockerCmd and DockerCmd instead of AsyncDockerCmd
* add docstrings
* change bad names and other PEP8/pylint changes

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
subtests.docker_cli.tag: Add double_tag variants

This test tries to tag the same image multiple times:
* without --force: Expected failure
* with --force: Expected pass

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Merge pull request #376 from ldoktor/docker_tag

 subtests.docker_cli.tag: Add double_tag variants and fix couple of bugs